Now click in the command window (lower right) and type "load_sky dsky".

The actual sky textures are up to six separate images that map onto a cube. The names of the images are the name of the sky plus "t" for top, "b" for bottom, and "n", "e", "s", "w", for north, east, south, and west. So, for dsky, the textures are:

dskyt.pcx

dskyn.pcx

dskye.pcx

dskys.pcx

dskyw.pcx

You will notice that dskyb.pcx (bottom) is not present. Any missing face is replaced with jorge. Shouldn't be a problem if terrain doesn't allow Garrett to see sky below a certain line on the horizon, but if you want to do that just whip up a dskyb.pcx for your mission.

This is the same way Quake 2 handles sky/horizon images. Here's a link with a tutorial (about halfway down) and some sample skies (environments):